Spring clutch mechanisms



` Jan. 17, 1961 c. R. sAccHlNl Erm. 2,968,380

SPRING CLUTCH MECHANISMS Filed 211. 30, 1957 INVENTOR. (2u unal/.r F.Sarra/.v1 Da/up Tan/ 0 HTT'GRNE l United States Patent "lee REISSUED 1AUG. 21, 1962 RE 25.229 z,9ss,ss0

I SPRING CLUTCH MECHANISMS Filed Jan. 30, 1957, Set'. No. 631,239

3 Claims. (Cl. 1921-81) Spring or coil clutch mechanisms when arrangedfor lil intermittent drive of a rotary load from a continuously urotating shaft or the like commonly have coaxial input and outputrelatively rotatable circular drums land a helical coil friction springis rcsiliently prcloaded at leas-ton the input drum and rettetivelyconnected to the output drum. Thereby the clutch spring is selfenergizing by friet on normally toy grip the two drums and drive theload. but the arrangement permits the input drum to overrun the springin frictional contact therewith (in other words permits effectualdisengagement of the clutch mechanism) when the coil of the spring whichis farthest from the output drum along the rotational axis (input free"end coil) is arreted. ln such spring clutches a control sleeve or collarloosely enveloping the spring and rotatable therewith is commonlyattached to the input-drum associated free enti coil of the spring, anda control finger or plunger engages a release shoulder on the sleeve todisengage the clutch.

ln ease the clutch spring is resilient or elustically preloadcd on theoutput drum aswell as on the input drum. but is otherwise unattached toeither drum. thus being, as commonly known in coil clutches, a fullfloating type clutch spring. then when the control sleeve or collar isarrested by the stop linger the output drum and meehanism connectedtherewith can overrun the clutch spring and come to a stop unrestrainedby the clutch mechanism except for the overrunning friction of thespring coils on the output drum. if on the other hand the clutch springis positively secured to the output drum and the load has considerableinertia Aand kinetic energy then, during the clutch `disengagingoperation. the spring eithertcnds to be injured by excessive expansionof its coils or, if expansion is limited as by radial abutment of itscoils. with the control sleeve, then the disengaging operation is likelyto be accompanied by undesirable shock tothe mechanism. Thus usually theclutch spring is preloadcd for necessary reactance and self energizationon the output drum as well as on the input drum. Heretofore. we believe,such preloading of the clutch spring on both the lnput and output drumsurfaces has been approximately uniform at least by intention or design.

When such uniform or nearly uniform preloading, as Y just above referredto. is practiced then during all operating times when the clutch isdisengaged the input-drum-associated coils ride the continuously rotatedinput drum with appreciable force (generally proportional to thepreloading); producing undesired temperaturerise. YMore important, inpracticallyall types of service, the inputdrum-associated coils tend tovibrate or flutter, producing so-ealled fretting or vibration corrosionof lthe coaeting coil and drum surfaces and premature wearing out of jthose surfaces.` .j j

We have discovered that 'by providing: a materially greater preloadingor interference fitting relationship bctween one or more coils lof thefull floating clutchsprng and the output drum than any coils'of'thsspring have in respect to the input drum then (assuming the kinetic2,968,38() Patented am. 17, 19st energy of the load is sutllcient as isalmost always true) all the lnput-drum-assoclated coils are moved, e.g.expanded, entirely clearof the input drum and so remain as long as theoutput drum is prevented from being turned backward following the clutchdisengaging operation and overrunning of the output drum relative to thespring. Average loads have sufilcient built-in friction and inertia toprevent retrograde motion of the output drum after it it comes to astop; and in exceptional cases or whenever required many simple buteffectual antirotational or noback ldevices are applicable to preventretrograde rotary motion following forward motion.

We have found further that by the use of a relatively simple formuladcrvied from well recognircd principles of helical spring or coil clutchconstruction and applicable to the materials and spring stock sectionshapes most etleetually used therein, the necessary or desirable amountt of diametrul clearance between the input drum and its assoeiated coilsduring clutch disengagement can be reliably predicted as' a function ofoverrunning torque resulting from selection of (a) suitable' numbers ofcoils relatively heavily elastically prcloaded on the output drum and(b) the degree or amount of elastic preloading of those coils on suchdrum. s

ln the accompanying drawing Fig. l is a double scale centrallongitudinal sectional assembly view of an illustrative embodiment ofthe present clutch mechanism. Fig. 2 is a full scale end elevation ofthe mechanism viewed as indicated on Fig. 1 by the line 2-2. Fig. 3 isfragmentary double scale sectional and elevational view of a preferredform of clutch spring. j

ln Fig. l. the input clutch drum member 1 is shown in the form of anadapter for a power shaft 2 assumed to have suitable bearing supportsnot shown. The drum member l is suitably secured as by a screw 3 to turncontinuously with the power shaft. Tite output drum member 5 is shown inthe form of a pulley supported for rotation relative to the shaft 2 anddrum member l as by a suitable bearing bushing 6 between a reduceddiameter surface portion 7 of the input drum member and an internalcircular surface 8 of the pulley hub (e.g. bushing press fitted in thepulley hub). The circular clutch drum surfaces 10 and 1,1 are preferablycylinders of equal diameter formed on the drum members l and 5respectively. The axially adjacent inner end surfaces of the members 1and 5, as shown, are maintained in closely spaced spart relationship asby a washer l2 between the drum members in cooperation with a snap ringand washer assembly 14 carried by a tubular portion of the'drum memberl, left, Fig. l. Axial shoulders 15 and`16 on the drum members 1 and 5adjacent respective drum surfaces 10 and l1 provide an annular channelfor the clutch spring 20. The channel, as shown, is partly occupied by arib portion 23 of a clutch control sleeve 22, described later, theinvolved parts being dimensioned to insure adequate free workingclearance for thel clutch spring 20 axially of the spring.

Clutch spring 20, as shown particularly in Fig. 3 in a relaxed state, isof generally cy'indricalv form with inputdrum-associated coils 20a andoutput-drum-a sociated coils 20h and 20d. The endmost free end coil 20ahas an outwardly projecting release toe 20e formed thereon. Por clutchdisengagement purposes the lcontrol sleeve 22 is axially slotted at 22ato receive the tee 20c, and the sleeve' 22 is supported for freerotationrelative to both clutch drum members l and! as on circular surfaces 1dand 19 of the drum members adjacent to their shoulders l5 and V16respectively. The `control sleevey 22 has one or more clutch releaseshoulders or abutments 24, two being shown, projecting radiallytherefrom for'engagement' by n stop pin or plunger 25 as partially shownin Fig. 2. The clutch mechanism is shown in Fig. 1 with the clutchspring 20 of Fig. 3 in torquetransmittng or engaged condition or withall its coils contracted against the drum surfaces. The stop pin 25 isprojected into the path of rotation of the shoulders or abutments 24 byany suitable means, not shown, in order to disengage the clutchmechanism by arresting rotation of the clutch spring.

Referring further to Fig. 3 the coils 20a of spring 20 are made withtheir normal inner diameters smaller than the external diameter of inputdrum surface 10 or so as to be in elastically or resiliently preloadedrelation thereto sufficiently to insure their self energization on theinput drum. Output-drum-associated coils 20b, as shown, have the samediameters as coils 20a. A preferably limited number ofoutputdrum-associated free end coils 20d of the spring, for examplethree free end coils, are formed (e.g. as part of the same automaticcoiling machine cperation which forms coils 20a and 20h) in radiallyoffset relationship to the other coils inwardly toward the coil axis soas to have materially greater elastic or resilient preloading on theoutput drum surface 11 than any of the other coils have on theircoacting drum surfaces. An increase in the number of coils 20d relativeto the number of coils Zlib at any particular preloading of coils 20dwill increase the overrunning torque. The number of coils 20d and theamount of radial ot'set of those coils (exaggerated in Fig. 3) areselected particularly in relation to the index of the spring (mean coildiameter divided by the radial dimension or height of the individualcoils) so that, assuming the output drum member 5 has substantial massor is connected to a body having substantial mass supported forrelatively free rotation, the operation of arresting the rotation of thecontrol sleeve to disengage the clutch mechanism will result in overrunof the output drum member 5 in frictional contact with the coils 20d,and the overrunning torque thereby applied to the clutch spring 20 will,in turn, result in expansion of its coils 20a completely clear of theinput drum sur face 10.

The coils 20a and 20b, incidentally, become approximately uniformlyexpanded by the overrunning action of drum surface 1l on the coils 20dso that the latter coils, by remaining in seated position on the drumsurface l1, tend to hold all the expanded coils in uniformly spacedrelationship to their associated drum surfaces or l1.

The reason for limiting the number of coils 20d in relation to thenumber of coils in the clutch spring which are associated with theoutput drum surface 11 is so that, when providing a minimum(axial-space-conserving) total number of coils in the spring in order tocarry the expected load, there will be a considerable number of thespring coils expanded during clutch disengagement as described above.Thereby if, after clutch disengagement, unusual conditions (e.g. lowfriction in the driven device or load) should permit back-travel orretrograde angular movement of the output drum following its overrun, agreater angular retrograde movement would be required in order to allowreseating of any of the spring coils on the input drum than would berequired if nearly all of the coils (20b and 20d) around the output drumsurface 1l were to have equal, relatively heavy, preloading on saidsurface. In other words the relatively lightly preloaded coils 20b, whenexpanded, augment the expanded coils 20a in safeguarding againstpossible backtravel as just discussed above.

The radial distance between the outer peripheral sr'- faces of thespring coils 20a and 20b and the inner peripheral surface 22b of therelease sleeve 22, when the coils are gripping, is preferably somewhatgreater than the distance the coils 20a and 20b are expanded when theoutput drum surface 1l frictionally overruns the relative- 1y heavilypreloaded coils 20d. Thereby clutch disengagement cannot possibly beaccompanied by shock or noise. By design, however, the spring index andthe to cause theoutput drum surface 11 to overrun the clutch springcoils 20d and with reference to the radial clearance between the coilsand the sleeve 22 when the coils 20a and 20b are in gripping position,so that torque of such magnitude will, during disengagement of theclutch, expand the input-drum-associated coils into light contact withthe interior surface of the sleeve 22, thereby further insuringcentering of all of the expanded coils with reference to theirassociated drum surfaces.

When a full floating type clutch spring, such as 20 y hereof Fig. l. isunconnected with its associated clutch drums except by elasticpreloading thereon, then some means is required to limit axial movementof the spring out of position. Shoulders similar to 15 and 16 of drummembers 1 and 5 hereof usually serve that purpose. We have found that ifthe free end of the clutch spring (e.g., the coil supporting the toe20c) which is associated with the input drum is permitted at times torub axially on a shoulder such as 15, which turns with the drum l duringclutch disengagement, then fretting corrosion and wear frequentlyresults, evidently because the rubbing contact produces vibration of theclutch spring or (por sibly) spasmodic contraction of the coils 20a intocontact with the input drum surface 10. Accordingly, as one convenientmeans for preventing contact between the spring and the input drummember 1 axially of the spring during clutch disengagement, the controlsleeve 22, which is constrained to rotate only when the clutch springrotates, is provided with an inwardly extending annular rib or ribportion 23 having a shoulder surface 23a axially adjacent the free endcoil of the clutch spring 20. Whenever the clutch spring moves or tendsto move toward the right (Fig. l) out of its normal or freeworkingposition, such movement is blocked by the rib 23 whose rightwardmovement, in turn, is blocked by the shoulder 15 of the input drummember l. The rib 23, during clutch disengagement, is normally out ofcontact with the shoulder 15 and it is never in tight contact therewith.

ln the herewith-illustrated full floating type spring clutchconstruction, as opposed to a construction in which the output end ofthe clutch spring is positively connected to the load. the maximumoverrunning torque which operates to expand the coils 20a completelyclear of the input drum surface l0 is automatically limited by themaximum static unit pressure the coils 20d can be caused to exertradially against the output drum surface 1l in being elasticallypreloaded thereon by interference fitting; and that unit pressure is. ofcourse, limited by the yield point of the spring material. Experiencehas demonstrated that if all the coils of the clutch spring have uniformresiliency (as when all the coils have uniform physical properties, andare of approximately uniform cross section) it is then impossible (inthe absence of auxiliary means) so to preload the coils 20d on theoutput drum as to produce, through overrun of the output drum, anexpansion of the coils 20a (and/or 20h) such as could injure those coilsalthough the mechanism has no means capable of limiting their expansion.Thus the internal surface 22b of the control sleeve 22 of the presentconstruction can be at any desired radial distance from the externalperipheral surfaces of the coils 22a and 22b when those coils arecontracted against their associated clutch drums.

Since the overrunning torque necessary to cause expansion of the coils20a out of contact with the drum surface 10 will depend upon (a) thespring index or the relationship between coil diameter and coil crosssection, (b) the coefficient of friction between output drum and clutchspring; and (c) the number of coils 20d in interference fitting relationto output drum surface l1, no universally applicable or simple rule ofprocedure or construction can be given such as would indicate theoperating limits or proportions which have to be observed in thesuccessful practice of the present invention. A working formula foruniform section spring stock la given naar the end of thisspecification.

We have fully tested the herewith illustrated spring clutch assemblyln'intermtttently driving a rotary fan at around i725 r.p.m. ln theinstallation tested the clutch drums have approxlmatelyequal diametersof 1313","

the nomai inner spring diameterat coils 20a and 20b is approximately1.290" and the normal inner spring diameter of coils 20d isapproximately 1.264", thus the interference fit or elastic preloading ofcoils 20a and 20h on the drum surfaces is approximately .023" whereasthe preloading ofthe coils 20d on output drum surface l! isapproximately .049". The spring 20 is of die drawn oil temperedsteelwire, A.l.S.l.e-l065, and has sn index of about 22.5. The torquerequired to cause overrun of the output drum member in the eolls 20d (inease member 5 is a zinc alloy die casting) is approximately 7.0inch-ounces. That torque expands the coils a and 20h so that their innerdiameters are approximately .022" larger than the associated drumdiameters or, in other words. the total expansion of those coils fromtheir normal or relaxed condition as shown in Fig. 3, is about .045".The coils 20a and 20h could withstand a much larger expansion withintheir elastic limits. Since, with the materials and dimensionalrelationships given above. more than sufficient separation of the coils20a from the input drum surface l0 is produced, it is apparent that thedifference in the elastic preloading of the coiia 20d and 20a on theirassociated drum could have been somewhat less and/or the spring indexcould have been somewhat lower.

In conducting the above described tests lt was found that friction inthe tan drive assembly, not shown, assisted by slight and practicallyunavoidable frictional drag in the bearings 6. 7. 8 was adequate alwaysto maintain the input-clutch-drum-associated colis 20a competely freefrom the drum surface l0 (during protracted clutch disengagement or idleperiods) despite the inherent tendency of the coils 20a and 20h toreseat themselves on the drums.

in an expanding-to-grlp type of spring clutch the coils corresponding to20d in Fig. 3 would be radially offset outwardly from the other coils inthe preferred practice of the present invention, and uneolllng" on partof the input-drum-associated coils would be a contraction of the coils.

Formula In any clutch spring made from constant cross section andotherwise uniform spring stock, overrunning torque (To) is the productof a value known in the art as overrunning torque ratio: (K0), or

(wherein "e" is the b'tse of natural logarithme or 2.7183; N" is thenumber of rolls in interference fit. and .f is the coefficient offriction) multiplied by so called energizing torque": (T.) or

(wherein "E" is the modulus of elasticity of the spring stock; 1" is themoment of inertia of the springatock section; A" is the diametraldeflection or interference tit of the spring coils on the drum. and D isthe mean diameter of the various coils).` Thus Toa-Kor..

Assuming in the present clutch mechanism that the elastic preloading ofcoils 20d is selected to produce sufficient overrunning torque to expandcoils 20a clear of the input drum during clutch disengagement it isevident that the overrunning torque must then equal the spring moment ofcoils 20a (and 20h). The desired diametrai clearance of the coils 20a inrelation to the input drum l0 can be thereby equated with the othernecessary design values or factors involved, as follows:

(wherein A," is such desired diametral clearance of coils 20a; "A," isthe diametral deflection or amount of interference fitting of coils 20don the output drum; 1(0" is the overrunning torque ratio as expressedabove (involvlng coils 20d) and "An" is the diametral deflection orenergizing interference fitting of coils 20a on the input drum 10). Thevalue An is subtracted, as indicated, because of the portion of thetotal overrunning torque which is required to obtain zero clearance orto overcome the initial preloading of coils 20e.

Regardless of the selected number of relatively heavily preloaded coils20d and the amount of interference tit they have on drum 1f (which is,of course, within the elastic limit or yield point of the springmaterial) the stress in the expanded coils 20a (or 20h) cannot exceedthe initial stress involved in the elastic preloading of coils 20d ondrum 11. This is believed evident from the formula for overrunningtorque ratio" given above. since its value must be some fraction of l.The fo'mula for spring moment for a given diametral deection is the sameas for energizing torque.

We claim:

i. A friction coil clutch mechanism for coupling a oontinuously rotatingdrive to an intermittently operated load, said mechanism comprising anapproximately eylindrical input clutch drum. an approximatelycylindrical output clutch drum coaxial therewith, a full-floating typehelical clutch spring having coils elastically preloaded radiallyagainst approximately cylindrical surface portions of respective drums,annular axial shoulders on respective drums facing respective end coilportions of the spring, a clutch release member and means supporting therelease member for rotation relative to the drums about their commonaxis and in a predetermined axial position in the mechanism. saidrelease member being connected to an input-drum-associnted end coil ofthe spring, for rotation integrally with the spring, means operable tostop rotation of the release member hence rotation of the spring todiaengage the clutch mechanism, said release member having a portioninterposed between auch input-drum associated free end `coil of thespring and the axial shoulder of the input drum.

2. A friction coil clutch mechanism for coupling a continuously rotutingdrive to an intermittently operated driven load, said mechanismcomprising an input clutch drum. an output clutch drum coaxialtherewith, a fulliloating type helical clutch spring huving coilselasticitily preloaded radially against respective drums. a clutchrelease abutment connected to an input-drum-associnted end coil portionof thc spring and facing circumferentially of the spring in its drivingdirection. and u stop disposed to engage the abutment to stop rotationof said end coil portion of the spring und thereby disengage thc clutchmechanism, the elastic preloading radial force of one or more of theoutput-drum-associuted coils of the spring being greater than theelastic preloading radiui force of any of the input-drum-ussocintedcoils of the spl-Ing, the input drum having un axially presentedshoulder facing the inpul-drumassociuted end coil of the spring, andsaid clutch mechanism including a member connected to the spring forrotational movement integrally therewith. said member having a portionlying between suid end coil and the shoulder.

3. A friction coil clutch mechanism for coupling a continuously rotatingdrive to an intermittently operated driven load. said mechanismcomprising an approximately cylindrical input clutch drum. anapproximately cylindrical output clutch drum coaxial therewith and ofapproximately the same diameter as the input drum, a fulloating typehelical clutch spring all the coils of which have substantially the samecross sectional dimensions.

the spring being elastically prelosded radially against redrum is on theorder of twice the elastic preloading speetive drums, a clutch releaseabutment connected to force of any coils of the spring on the inputdrum.

an input-drum-associated end coil portion of the spring and facingcircumferentially of the spring in its driving l.

direction, and a stop disposed to engage the abutment I Mmm Cmd in n me0f this Pacn to stop rotation of said end coil portion of the spring andthereby diseugage the clutch mechanism, there being UNITED STATESPATENTS a dierence in diameter between coils at opposite end 390,029Lombard Sept. 25, 1888 portions of the spring when the spring is in arelaxed l 1,641,418 Elsey Sept. 6, 1927 state such that, in operatingposition, the elastic preload- 10 2,298,970 Russell et al. Oct. 13, 1942ing force of at least one coil of the spring on the output 2,685,949Dunlap Aug. 10, 1954

